The invention is based on an electromagnetically actuated valve, particularly for slip-controlled hydraulic brake systems in motor vehicles as defined hereinafter.
A valve of this kind is known by means of German Patent 39 34 771 C1, FIG. 3. It has a control piston disposed in the valve dome which piston forms a stop for the magnetic armature and which is guided so that it can slide longitudinally on a tang, which leads from the valve body and penetrates the magnetic armature. The control piston defines with its base remote from the armature a control chamber, which communicates with the pressure fluid inlet of the known valve by means of a pressure fluid conduit which penetrates the tang and the control piston on the same axis. While the magnetic armature is pressure balanced on both face ends, controlled pressure introduced into the control chamber can slide the control piston axially against a stop. By means of this, the stroke of the magnetic armature is reduced by a predetermined measure, which results in a throttling of the through flow cross section.
This manner of function of the known valve is useful in slip-controlled hydraulic brake systems in which the pressure fluid inlet communicates with the master cylinder and the pressure fluid outlet communicates with a brake cylinder. In a controlling of brake slip for example, the valve is switched into its closed position by means of exciting the magnet coil and upon reduction of the pressure in the brake cylinder, produces a high enough pressure difference between the pressure fluid inlet and the pressure fluid outlet, so this leads to the above mentioned sliding of the control piston with the result that upon opening the valve, the previously described throttling of the through flow cross section is in effect, as long as the pressure difference between inlet and outlet exists. Because of the reduced pressure gradient during the pressure buildup of a controlling of brake slip that follows a pressure decrease, the reduction of the through flow cross section has a positive effect on the standard quality and noise behavior of the brake system. In a normal braking without danger of locking, in comparison, the entire through flow cross section of the valve is available, which expedites a desired short reaction time of the brake system upon actuation of the brakes.
With the known valve, however, the fixed adjustment of the throttled through flow cross section is disadvantageous because by means of this, the through flow quantity is subject to fluctuations which depend on the pressure differential. Besides, the through flow quantity is in essence completely dependent upon the absolute measure of the through flow cross section, i.e. the stop requires a very narrow tolerancing. Further, the valve is relatively expensive to produce because of the control piston it requires.